Reduce process duration of test_Http2FrequencyCounter

[masaori335]

Prior to this change, test_Http2FrequencyCounter took almost 2 mins to run. Because there were 4 loops like below. ( the expected value of each loop is 29.5 sec ).

 while (now % 60 != 0) {
	 sleep(1);		
	 now = ink_hrtime_to_sec(Thread::get_hrtime_updated());		
}

[zwoop]

Cherry-picked to v9.0.x branch.